1. Field of the Invention
The present invention relates to a bearing alloy for high-temperature application which is used in an oxidizing atmosphere at high temperature.
2. Technical Background
Conventional bearings for high-temperature application, especially bearings in exhaust gas control valves of gasoline or diesel engines, variable valves of gas turbines, and so forth, are used in an oxidizing atmosphere at a temperature exceeding 700.degree. C. There has been an increasing demand for a bearing alloy which can be suitably used in an oxidizing atmosphere at such a high temperature. In order to satisfy the demand, the inventors of the present application suggested a bearing alloy which can be used even at 900.degree. C., as disclosed in JP-A-5-17839.
The known bearing alloy consists of, by weight, 9 to 30% Cr, 5 to 19% Fe, 0.1 to 1.5% Si, 2 to 22% Co, 1.4 to 11% Mo, and the balance of Ni and unavoidable impurities, and has a structure in which hard particles of a Co--Mo--Cr--Si-system intermetallic compound are dispersed in the alloy matrix. Further, according to JP-A-5-17839, in order to improve oxidation resistance of the known bearing alloy, it is essential to sinter a green compact of a powder thereof and subsequently hot-compress it to increase the density thereof. When the mating material is an Ni-base heat resisting alloy of the precipitation hardening type, the bearing alloy exhibits excellent sliding properties because of the hardness as high as Hv 300 to 500. However, for example, when a soft mating material such as austenitic stainless steel of Hv 100 to 300 is used, wear loss of the mating material increases in spite of the fact that the friction coefficient is low, which results in a problem of deterioration in durability owing to wear of the mating material. Moreover, in the manufacturing process of the known bearing alloy, the density must be increased by hot-compressing so as to improve oxidation resistance, which makes the process complicated.
It is therefore an object of the present invention to provide a bearing alloy for high-temperature application which scarcely wears a mating material even if the mating material is soft and has a hardness not higher than Hv 300, which bearing alloy has excellent oxidation resistance without increasing the density by hot-compressing and can be easily manufactured.